A variety of polymerization processes are used in the formation of synthetic polymers. For example, the polymerization of a monomer can be conducted by bulk polymerization, suspension polymerization, emulsion polymerization or solution polymerization. Each of these systems has certain advantages and disadvantages.
Bulk polymerization is the direct conversion of liquid monomer to polymer in a reaction system in which the polymer generally remains soluble in its own monomer. Such bulk polymerizations are generally carried out by the addition of an initiator to a simple homogeneous system containing one or more monomers. The synthesis of polystyrene by the addition of a free radical initiator to styrene monomer is a good example of a very common bulk polymerization. The principal advantage of a bulk polymerization process is that the product obtained can be used directly since it is essentially free of solvents and impurities. One disadvantage of bulk polymerization reactions is that it is difficult to control the reaction temperature during polymerization.
In suspension polymerization, the initiator is dissolved in the monomer, the monomer is dispersed in water, and a dispersing agent is added to stabilize the suspension formed. All suspension polymerization processes use some type of surfactant and/or dispersing agent to keep the monomer globules dispersed through the reaction mixture in order to avoid coalescence and agglomeration of the polymer. Not only does the suspension stabilizer affect the particle size, but also the clarity, transparency and film-forming properties of the resultant polymer. A variety of dispersing agents including water-insoluble, finely divided, inorganic materials and organic materials, depending upon the monomer to be polymerized, have been used as dispersing agents. Thus, for example, talc, barium, calcium and magnesium carbonates, silicates, phosphates and sulfates, as well as poly(vinyl alcohol) tragacanth gum, salts of styrene-maleic anhydride copolymers, vinyl acetate-maleic anhydride copolymers and their salts, starch, gelatin, pectin, alginates, methyl cellulose, carboxymethyl cellulose, bentonite, limestone and alumina have been used as suspending agents. A major advantage of suspension polymerization is that the polymeric products are obtained in the form of small beads which are easily filtered, washed and dried. For reasons of cost and unreactivity, water is often a much more desirable diluent and heat-transfer medium than most organic solvents.
However, in certain polymerization processes, for example, in the preparation of very high cis-1,4-polybutadiene with catalyst systems based on nickel compounds the presence of any moisture is highly undesirable. Thus, suspension polymerization in a water medium is not an effective process for the synthesis of very high cis-1,4-polybutadiene utilizing nickel catalyst systems.
An emulsion polymerization process is considered to be a three-phase reaction system consisting of large droplets of the monomer, the aqueous phase containing the dissolved initiator, and the colloidal particles of monomer-swollen polymer. While the emulsion polymerization process has the economic advantage of using water as the emulsion base, not all polymerization processes can tolerate the presence of water.
Such is the case with the polymerization of butadiene into very high cis-1,4-polybutadiene utilizing nickel catalyst systems.
In solution polymerization, an organic solvent is used which is capable of dissolving the monomer, the polymer and quite often the polymerization catalyst or initiator. Inasmuch as the polymer is soluble in the organic solvent which is used, there is a tendency for the viscosity of the solution to increase as the molecular weight of the polymer increases. If this continues over a period of time, the solution becomes too viscous to handle in conventional polymerization reaction systems unless the solids content is limited to a low level. In commercial polymerization processes, it is desirable to obtain a polymerization mass which has a high concentration of solid polymer and at the same time which is easy to handle and does not accumulate on the walls of the reaction vessel.
A process for the nonaqueous dispersion polymerization of butadiene monomer into a very high cis-1,4-polybutadiene would be very desirable. Such a nonaqueous dispersion polymerization process generally offers several distinct advantages over other possible polymerization techniques, including improved heat transfer, higher polymer concentrations in the reaction medium, increased production capacity, and energy saving.
A process for the nonaqueous dispersion polymerization of butadiene monomer in a liquid nonaqueous dispersion medium, for instance, n-butane or n-pentane with a Ziegler-Natta catalyst which utilizes a block copolymer dispersion stabilizer is described in U.S. Pat. No. 4,098,980 assigned to The Goodyear Tire & Rubber Company. This patent is hereby incorporated by reference in its entirety. The block copolymer dispersion stabilizer utilized in U.S. Pat. No. 4,098,980 contains at least two blocks of polymers linked by chemical valences, at least one block (A block) is soluble in liquid organic dispersion medium and at least another block (B block) is insoluble in the dispersion medium and the stabilizer acts to disperse the polybutadiene which is formed in the stabilizer's presence.